What everyone says. Plus a lot of low sellers are probably the sort of nice photos that 'casual browsers' who would never buy like to have a look at, whereas lots of the high sellers aren't really of interest to any but the buyers (business shots etc.)
I hadn't thought of spider bots counting as a view. Anyone know if that's the case on iStock? Could explain a lot if so! E.g. I haven't had a sale since Saturday, but as it happened I fetched stats via DM before I went to bed and then again now and have had 318 files viewed in 9 (overnight, GMT) hours (and no sales) but noticed a lot of views whizzing through for my Christmas and Chanukah files (which are linked on iStock, so would presumably spider out).